Electronic structure and vacancy formation in photochromic yttrium oxy-hydride thin films studied by positron annihilation

M.P Plokker,S.W.H Eijt,F Naziris,H Schut,F Nafezarefi,H Schreuders,S Cornelius,B Dam

doi:10.1016/j.solmat.2017.03.011

Abstract

In order to investigate the mechanism of the photochromic effect in yttrium oxy-hydride (YOxHy) thin films, Doppler broadening positron annihilation spectroscopy (PAS) was applied to probe the electronic structure and the presence of vacancies in YOxHy and related materials as a function of composition, UV illumination and thermal annealing. The Doppler S and W parameter depth profiles of a series of Y, yttrium di-hydride YH1.9+δ and Y2O3 thin films show strong systematic changes caused by the distinct differences in electronic structure of the metals Y, YH1.9+δ and the wide band gap insulator Y2O3. The Doppler broadening parameters of photochromic YOxHy (a semiconductor with a band gap of ~2.6eV) are intermediate to those of YH1.9+δ and Y2O3. In order to probe the nanostructural changes related to the photochromic effect, the S parameter of YOxHy was monitored during in-situ UV illumination. A small but systematic increase of the S parameter was observed, possibly induced by generation of cation mono-vacancies or small vacancy clusters involving generated anion vacancies. The changes did not relax during bleaching under dark conditions, showing that the structural changes are not directly responsible for the photochromic mechanism. For temperatures above around 90°C, thermal annealing leads to a substantial increase in the Doppler S parameter, pointing to the formation of vacancies by local removal of hydrogen. Simultaneously, the optical band gap increases, consistent with an increase in O:H ratios.

Highlights

Inorganic materials such as the transition metal oxides WO3 and MoO3 show a photochromic effect [1] where visible or ultraviolet (UV) illumination leads to a coloration of the initially transparent films, closely related to that observed in the field of electrochromic materials [1,2,3,4]
In order to investigate the mechanism of the photochromic effect in yttrium oxy-hydride (YOxHy) thin films, Doppler broadening positron annihilation spectroscopy (PAS) was applied to probe the electronic structure and the presence of vacancies in YOxHy and related materials as a function of composition, UV illumination and thermal annealing
The presented positron Doppler broadening study showed that the electron momentum distribution changes substantially in the sequence Y → YH1.9+δ →Y2O3, reflecting the differences in the electronic structure of a metal with a free-electron-like contribution, a metal with covalent-ionic Y–H bonds, and an ionic wide band gap insulator, respectively

Summary

Introduction

Inorganic materials such as the transition metal oxides WO3 and MoO3 show a photochromic effect [1] where visible or ultraviolet (UV) illumination leads to a coloration of the initially transparent films, closely related to that observed in the field of electrochromic materials [1,2,3,4]. In the latter, the coloration is linked to the insertion of H+ or Li+ ions, introducing additional electronic states within the forbidden gap which act as coloration centers [2,4]. UV illumination of yttrium hydride YHx leads to persis-

Methods

Results

Conclusion